The constant velocity joint is used to transmit the rotational force taken out of the engine through the transmission.
JP Patent Publication KOKAI NO. H7-317754 discloses a conventional rolling bearing unit for vehicle wheel having a constant velocity joint, where the constant velocity joint is combined with the rolling bearing unit, so that the driven wheel is rotatably supported by the suspension while the rotational force is transmitted to the driven wheel.
FIG. 8 shows the conventional structure disclosed in this publication. An outer ring or race 1 is supported by the suspension so as not to rotate when incorporated in the vehicle, and formed with a first mount flange 2 on its outer peripheral surface to be supported by the suspension and with outer ring raceways 3 in double rows on its inner peripheral surface. A hub 4 is provided on the radially inside of the outer ring 1, and formed with a second mount flange 5 on its outer peripheral surface at the axially outer end (left end in the drawings) to support a vehicle wheel (not shown), with inner ring raceways 6 in double rows on its outer peripheral surface at the axially middle portion, and with a housing 8 for the constant velocity joint 7 at the axially inner end (right end in the drawings). A plurality of rolling members 9 are provided between the outer ring raceways 3 and the inner ring raceways 6 to rotatably support the hub 4 inside the outer ring 1.
The terms "axially outer" and "axially outside" mean the widthwise outside when installed in the vehicle while the terms "axially inner" and "axially inside" mean the widthwise inside when installed in the vehicle in the present specification.
Substantially cylindrical covers 10 made of a metal plate such as stainless steel and annular seal rings 11 made of elastic member such as elastomer e.g. rubber are provided between the opposite opening portions of the outer ring 1 and the outer peripheral surface at the middle portion of the hub 4. The covers 10 and seal rings 11 isolate the portion where the rolling members 9 are located, from outside, and prevent the grease in this portion from leaking out to the outside, and the foreign matter such as rain water, dust from entering this portion.
The constant velocity joint 7 comprises, in addition to the housing 8, an inner ring or inner joint member 12 and balls 13, which are a rotation or torque transmitting member, respectively. The balls 13 are rotatably supported by a cage 14. Engaged in a spline joint with the inside of the inner ring 12 is the end of the drive shaft (not shown) which is rotated through the transmission by the engine.
Formed on the outer peripheral surface of the inner ring 12 are a plurality of (e.g. six) inner engagement grooves 15 in arcuate cross section which are circumferentially arranged with a uniform interval to extend at right angles with reference to the circumferential direction.
Formed on the inner peripheral surface of the housing 8 facing the inner engagement grooves 15 are a plurality of outer engagement grooves 16 in arcuate cross section which are circumferentially arranged to extend at right angles with reference to the circumferential direction.
The balls 13 can roll along the inner and outer engagement grooves 15, 16 when supported by the pockets 17 of the cage 14, respectively.
When installing into the vehicle the rolling bearing unit integral with the constant velocity joint as constructed above, the outer ring 1 is supported by the suspension through the first mount flange 2 while for example the front wheel, that is a driven wheel, is connected to the hub 4 through the second mount flange 5. The end portion of the drive shaft (not shown) is engaged in a spline joint with the inside of the inner ring 12 of the constant velocity joint to be rotatably driven by the engine through the transmission.
When the vehicle is moving, the rotation of the inner ring 12 is transmitted to the hub 4 through the balls 13 to rotate the driven wheel, e.g. front wheel.
There is a problem that the conventional rolling bearing unit for vehicle wheel as shown in FIG. 8 is hard to make the unit compact and lightweight. The reason is as follows;
In the constant velocity joint 7 integrally combined with the rolling bearing unit, six inner engagement grooves 15, six outer engagement grooves 16 and six balls 13 are used. In this case, in order to transmit the required torque keeping the rolling fatigue life of the inner engagement grooves 15 and outer engagement grooves 16 and of the rolling contact surfaces of the balls 13 of the constant velocity joint, the outer diameter of the balls 13 of the constant velocity joint 7 must be large in a degree. Accordingly, in the conventional rolling bearing unit for the vehicle wheel, the diameter of the circumscribing circle of the outer engagement grooves 16 determined by the size of the diameter of the circumscribing circle of the balls 13 is larger than the diameter of the inner ring raceways 6 on the outer peripheral surface at the middle portion of the hub 4.
Therefore, in the conventional rolling bearing unit for the vehicle wheel, the rolling bearing section comprising the outer ring 1, hub 4 and rolling members 9 and the section of the constant velocity joint 7 are arranged in series in the axial direction as shown in FIG. 8. However, in this series arrangement, the whole axial size of the rolling bearing unit for the vehicle wheel is larger, and by that amount, the whole apparatus weight is larger. The weight increase of the rolling bearing unit increases the unspring weight of the vehicle, which would worsen the ride comfortability and fuel consumption performance. Therefore, making the rolling bearing unit for the vehicle wheel compact and lightweight is required.
Under such a situation, the present inventors invented the rolling bearing unit for the vehicle wheel combined with the constant velocity joint disclosed in JP Patent Application No. H9-191433.
In the rolling bearing unit for the vehicle wheel combined with the constant velocity joint of this patent application, as shown in FIG. 9, the number of the inner engagement grooves 15 on the outer peripheral surface of the inner ring or inner joint member 12, the outer engagement grooves 16 on the inner peripheral surface of the housing 8a of the constant velocity joint 7a, which is provided on the axially inner end of the hub 4a, and the balls 13 between the inner and outer engagement grooves 15 and 16 are seven or more (e.g. 8-12), respectively, so that the load applied to the balls 13 during use of the rolling bearing unit is smaller than in the conventional structure. By that amount, the outer diameter of the balls 13 is made smaller, so that the diameter of the circumscribing circle of the balls 13 arranged in an annular shape, and the diameter of the circumscribing circle of the outer engagement grooves 16 are made smaller than the diameter of at least the axially inside one of the inner ring raceways 6 which are formed on the outer peripheral surface of the middle portion of the hub 4a. In addition, the axially inside one of the inner ring raceways 6 is made to overlap part (the left part in FIG. 9) of the outer engagement grooves 16 in the radial direction.
The operation to rotatably support the vehicle wheel to the suspension by the rolling bearing unit of this application is substantially the same to that of the conventional rolling bearing unit as mentioned above.
Particularly, in the case of the rolling bearing unit of this patent application, the diameter of the circumscribing circle of the outer engagement grooves 16 is smaller than the diameter of the axially inside one of the inner ring raceways 6, so that this inner ring raceway 6 is made to radially overlap part of the outer engagement grooves 16. Accordingly, by this overlap amount, the axial size of the rolling bearing unit for the vehicle wheel is made smaller so as to make the whole apparatus compact and lightweight.
However, it is difficult to incorporate in the structure of FIG. 9 the tone wheel for detecting the rotational speed of the vehicle wheel. Specifically, detecting the rotational speed of the vehicle wheel is required to control the antilock brake system (ABS) and the traction control system (TCS). And, the tone wheel with the circumferential property changed alternately with a uniform interval must be fixedly supported on the inside of the first mount flange 2 on part of the hub 4a rotating with the vehicle wheel in order to detect the rotational speed.
However, in the case of the patent application of FIG. 9, the inner ring raceway 6 on the axially inside is made to radially overlap part of the outer engagement grooves 16 to reduce the axial size of the rolling bearing unit, which reduces the axial size L.sub.18 of the axially inner end portion 18 axially inwardly projecting from the seal ring 11 at the axially inner end of the housing 8a adjacent the axially inner end portion of the hub 4a.
Fitted onto the axially inner end portion 18 is the axially outer end portion of the boot (not shown) which shields the interior of the housing 8a from outside to prevent the foreign matter from entering the housing 8a while preventing the grease in the housing 8a from leaking out. In addition, the axially outer end of the boot must be retained on the outer peripheral surface at the axially inner end portion of the housing 8a with a retaining band (not shown). For the rolling bearing unit combined with the constant velocity joint improved as shown in FIG. 9, the space for incorporating the tone wheel is not available so long as any modification is made.